Lead is a heavy metal which is toxic to mammals if it is ingested in a form in which it can be metabolized and absorbed in the body, i.e., when it is ingested in a bioavailable form. The extensive use and widespread disposal of lead in the environment has contaminated many soils and wastes, as well as water contacting those soils and wastes. Thus, it is important to develop methods for minimizing lead solubility and for minimizing the bioavailability of lead from the environment.
Currently, many technologies are employed to clean up contaminated soils and wastes including thermal, biological, and physical/chemical treatments. These techniques generally require removing the contaminated soil, treating it, and either replacing it on-site or disposing of it away from the area of contamination. Such removal technologies are costly to practice, and destructive to the sites from which wastes are removed. Furthermore, because of the bulk of wastes that must be removed, and the presence of lead in such wastes, application of removal technologies presents a disposal problem when the removed materials are disposed of away from the site of contamination. In addition, these removal technologies are often ineffective in fully removing heavy metals or reducing their bioavailability.
In order to address problems of cost, unsightliness, and waste volume associated with removal technologies, in-place or in-situ treatment technologies have also been developed. Some of these include: 1) flushing of the soil with fluids that dissolve the lead; 2) immobilization of the lead in place by sorption or ion exchange of lead onto materials introduced into the waste; 3) precipitation of the lead in an insoluble form; 4) degradation of the lead-containing materials by chemical or biological techniques, such that the lead is solubilized, followed by removal of the solubilized lead; or 5) attenuation of the lead by addition of inert materials to the lead-contaminated waste. However, these existing in-situ technologies are generally either expensive or ineffective in removing heavy metals or reducing their bioavailability. For instance, soil flushing creates a high volume of lead-containing liquid that must be treated and disposed elsewhere. Also, many of the existing immobilization and attenuation technologies require the addition of large quantities of material that lead to unsightliness at the treatment site. Furthermore, many of the existing in-situ technologies involve several complex and time consuming treatment steps. Finally, many of the existing in-situ technologies involve chemical reactions that proceed with difficulty, very slowly, or inefficiently when applied at a contaminated site.
It has been suggested that phosphate minerals have the potential to immobilize lead. Nriagu, 11 Inorg. Chem. 2499 (1972); 37 Geochim. Cosmochim. Acta 367 (1973); 37 Geochim. Cosmochim. Acta 1735 (1973); and 38 Geochim. Cosmochim. Acta 887 (1974). Nriagu theorizes that lead is immobilized by phosphate due to the low solubility of lead orthophosphates, e.g., hydroxypyromorphite Pb.sub.10 (PO.sub.4).sub.6 (OH).sub.2 !.
The use of synthetic hydroxyapatite to remove lead from an aqueous solution has been disclosed. Y. Takeuchi and H Arai, 21 J. Chem. Eng. Jpn. 98 (1988) summing up work from 1981 to 1988). However, Takeuchi and Arai did not report the final aqueous lead concentrations in the solutions tested, so that the effectiveness of their method for removing lead could not be evaluated. Furthermore, Takeuchi and Arai did not investigate the use of synthetic hydroxyapatite to immobilize lead in lead contaminated solid materials.
Therefore, although use of phosphate minerals for immobilizing lead has been suggested in the prior art, there has been no prior disclosure of a method for accomplishing immobilization of lead in lead-contaminated solids such as wastes and sediments using solid calcium phosphate-containing materials.